Lightweight thermal management material for enhancement of through-thickness thermal conductivity

ABSTRACT

A flexible sheet of aligned carbon nanotubes includes an array of aligned nanotubes held in a polymer matrix material. The carbon nanotubes have an average length of between about 50 microns and about 500 microns. The polymer matrix has an average thickness of between about 10 microns and about 500 microns. The flexible sheet has a density of about 0.2 to about 1.0 g/cc and includes between about 98 to about 60 weight percent aligned carbon nanotubes and between about 2 and about 40 weight percent polymer. A tape of aligned carbon nanotubes, a method for producing a tape of aligned carbon nanotubes, a method of producing the flexible aligned carbon nanotube sheet material and a method of increasing unidirectional heat conduction from a work piece are also disclosed.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

The present invention relates generally to thermal management materialsand, more particularly, to thermal management materials incorporatingaligned carbon nanotubes held in a polymer matrix and methods forproducing such materials.

BACKGROUND OF THE INVENTION

A carbon nanotube is a microscopic, hollow filament comprised of carbonatoms arranged in the shape of a cylinder. Carbon nanotubes aretypically on the order of nanometers in diameter but may be producedwith lengths of up to several hundred microns. Carbon nanotubes possesshigh electrical and thermal conductivities in the direction of thelongitudinal axis of the carbon nanotubes. Individual carbon nanotubeshave displayed thermal conductivities of 3000 W/m−° K and higher at roomtemperature.

It is known to use composites of aligned carbon nanotubes and a polymermatrix in thermal management applications. Examples of such compositeused for thermal applications are found in U.S. Pat. No. 6,428,890 toTing and in published U.S. Patent Applications 2006/0279191 to Goheganet al and 2007/0116626 to Pan et al. The present invention relates toimproved composite materials made from carbon nanotubes and a polymermatrix including continuous tapes of such material as well as to methodsof their production. In addition, the present invention relates to amethod of increasing unidirectional heat conduction from a work piece.

SUMMARY OF THE INVENTION

In accordance with the purposes of the present invention as describedherein, a method is provided for producing an aligned carbon nanotubetape. The method comprises the steps of preparing strips of alignedcarbon nanotube arrays and splicing those strips of aligned carbonnanotube arrays together end-to-end on a flexible support to form atape. In addition the method includes covering the tape with a peel plyso that the spliced strips are covered on a first face by the supportand on a second, opposite face, by the peel ply. The method alsoincludes the step of winding the covered tape into a roll.

More specifically describing the invention, the preparing step includesthe steps of growing an array of aligned carbon nanotubes on a substrateand fixing the aligned carbon nanotubes in alignment. The fixing stepspecifically includes the steps of infiltrating the array of alignedcarbon nanotubes on the substrate with a matrix material such as apolymer and allowing the polymer to partially cure.

In addition, the preparing step may further include the steps ofslitting the polymer-infiltrated aligned carbon nanatube arrays intostrips of width W and removing strips of width W from the substrate.Alternatively the multiwalled carbon nanotubes can be grown onsubstrates of pre-determined width W, and then infiltrated with polymer.The removing step may be performed by shaving away the carbon nanotubesfrom the substrate or etching away the substrate. Or, the substrate maybe left in place.

In accordance with an additional aspect of the present invention, a tapeof aligned carbon nanotubes is provided. The tape comprises a nanotubelayer including aligned carbon nanotubes held in a polymer matrix. Aflexible support covers the first face of the nanotube layer. Inaddition, a peel ply covers a second opposite face of the nanotubelayer. In one possible embodiment the aligned carbon nanotubes aremulti-walled carbon nano tubes.

The matrix used in the tape may be made from a material selected from agroup consisting of any of a series of thermosetting or thermoplasticresins, including epoxy, vinyl ester, silicone, cyanate ester,bismaleimide (BMI), polyimide, polyolefin, polyurethane, phenolics,acrylics, polyester; carbonizable resins such as polyfurfural, pitch, ortars; ceramic or metallic matrices including, silicon carbide, aluminum,and solders; or rubbers and mixtures thereof. The flexible support maybe made from a material selected from a group consisting ofpolytetrafluoroethylene (PTFE), fiber reinforced PTFE sheet, polyester,polyolefins, coated paper, coated fabric, wax, silicone flexible metalsor rubbers and mixtures thereof. The peel ply may be made from amaterial selected from a group consisting, of polytetrafluoroethylene,polyester, nylon, coated paper, coated fabric, silicone, wax,polyolefin, metals or rubbers and mixtures thereof. An adhesive layermay be provided between the nanotube layer and the peel ply. Thatadhesive is selected from a group consisting of thermosets, latexs,rubbers, acrylics, pressure sensitive adhesives, silicones or othernatural or synthetic adhesives and mixtures thereof.

In accordance with yet, another aspect of the present invention, amethod is provided for increasing unidirectional heat conduction from awork piece. That method may be broadly and generally described ascomprising the step of wrapping the work piece with a continuous tape ofaligned carbon nanotubes. That continuous tape of aligned carbonnanotubes includes a support layer, a nanotube layer and, optionally, apeel ply layer. Accordingly, the method may be more specificallydescribed as including the steps of removing the peel ply layer from thetape (if applicable), winding the tape around the work piece with thenanotube layer wound around and engaging the work piece and thenunwrapping the support layer from the nanotube layer.

In one possible embodiment, the method further includes providing anadhesive between the nanotube layer and the work piece. In any of theembodiments, the method may further include the curing of the continuoustape following the unwrapping of the support layer. Finally, the methodmay include the selecting of the work, piece from a group of structuresincluding a rocket motor casing, a microprocessor chip, laser equipment,filament-wound composites, laminate composites, electronics componentsfiber reinforced composites, sheet molded materials, composite enginecowlings and planar interfaces between heat sources and heat sinks.

In accordance with yet another aspect of the present invention a methodis provided of producing a flexible aligned carbon nanotube sheetmaterial. The method may be broadly described as comprising the steps ofsynthesizing an array of aligned carbon nanotubes on a substrate,infiltrating that array of aligned carbon nanotubes with a polymer,allowing the polymer to partially cure or solidify and removing theflexible aligned carbon nanotube sheet material from the substrate.Further, the invention includes a flexible sheet of aligned carbonnanotubes comprising an array of aligned carbon nanotubes held in apolymer matrix material wherein the carbon nanotubes have an averagelength of between about 50 micron and about 500 micron, the polymermatrix has an average thickness of between about 10 micron and about 500micron and the flexible sheet has a density of between about 0.2 toabout 1.0 g/cc and includes between about 98 to about 60 weight percentaligned carbon nanotubes and between about 2 and about 40 weight percentpolymer. In one particularly useful embodiment of the present invention,the polymer matrix is a B-staged epoxy.

In another useful embodiment the flexible sheet of aligned carbonnanotubes includes a peel ply covering a first face of the alignedcarbon nanotubes held in the polymer matrix.

In the following description there is shown and described severaldifferent embodiments of this invention, simply by way of illustrationof some of the modes best suited to carry out the invention. As it willbe realized, the invention is capable of other different embodiments andits several details are capable of modification in various, obviousaspects all without departing from the invention. Accordingly, thedrawings and descriptions will be regarded as illustrative in nature andnot as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings incorporated herein and forming a part of thespecification, illustrate several aspects of the present invention andtogether with the description serve to explain certain principles of theinvention. In the drawings:

FIG. 1 is a partially schematical side elevational view illustrating theformation of aligned carbon nanotubes on a support substrate;

FIG. 2 is a view similar to FIG. 1 but illustrating the infiltration ofthose aligned carbon nanotubes with a polymer matrix material;

FIG. 3 is a view similar to FIGS. 1 and 2 but illustrating the removalof the substrate and the formation of a flexible sheet of aligned carbonnanotubes;

FIG. 4 is a view similar to the earlier Figures but illustrating theformation of a continuous tape from strips of the flexible sheet ofaligned carbon nanotube material;

FIG. 5 is a view similar to FIG. 4 but showing the addition of anoptional peel ply layer to the continuous tape;

FIG. 6 is a perspective view of an aligned carbon nanotube tape in theform of a roll; and

FIGS. 7A and 7B are schematical side elevational views illustrating thewinding of the tape onto a work piece.

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Reference is now made to FIG. 1 illustrating an array of aligned carbonnanotubes 12 as grown and supported on a support substrate 14. Thealigned carbon nanotubes may be grown in accordance with procedures wellknown in the art. One particularly useful process for the growing ofaligned carbon nanotubes is set forth and described in U.S. Pat. No.7,160,531 to Jacques et al. This patent is owned by the assignee of thepresent invention and the full disclosure made in that patent is herebyincorporated into this document by reference.

FIG. 2 illustrates how the individual nanotubes of the array 12 arefixed in alignment. Specifically, one face 16 of a flexible support 18is coated in a polymer material 20. The flexible support 18 may be madefrom any appropriate material including but not limitedpolytetrafluoroethylene (PTFE), fiber reinforced PTFE sheet, polyester,polyolefin, coated paper, coated fabric, wax, silicone flexible metal orrubber and mixtures thereof. The polymer material layer 20 may be madefrom any appropriate polymer matrix material including but not limitedto any of a series of thermosetting or thermoplastic resins, includingepoxy, vinyl ester, silicone, cyanate ester, bismaleimide (BMI),polyimide, polyolefin, polyurethane, phenolic, acrylic, polyester;carbonizable resin such as polyfurfural, pitch, or tar; or rubber andmixtures thereof. Typically, the flexible support 18 has a thickness ofbetween about 100 microns and about 1 mm while the polymer layer 20 hasa thickness of between about 10 microns and about 500 microns. Aftercoating, the face 16 of the flexible support 18 coated in the polymermaterial 20 is placed over the exposed face of the aligned carbonnanotubes of the array 12 in order to infiltrate the array with thepolymer (see. FIG. 2). Appropriate pressure may be applied to force thepolymer layer 20 down into the array 12 of carbon nanotubes. Once thespace between the carbon nanotubes in the array 12 has been sufficientlyinfiltrated with the polymer 20, the polymer is allowed to partiallycure and/or solidify. Partial curing is commonly known as B-staging. Theresulting polymer layer 20 holds the carbon nanotubes in the array 12 inalignment.

The next steps of the process are illustrated in FIG. 3. After partialcuring or solidifying of the polymer layer 20, the non-stick supportlayer 18 is peeled away. This is followed by the separating or removingof the substrate 14 from the flexible aligned carbon nanotube sheetmaterial 22 that comprises the carbon nanotube array 12 held in thepolymer layer 20. This may be done by a number of processes includingbut not limited to shaving the array 12 from the substrate 14 andetching the substrate away with an acid. In the shaving process a sharpblade is held at a shallow angle to and against the surface 24 of thesubstrate 14 (see FIG. 1). As the blade is pushed forward, the carbonnanotubes of the array 12 are cut free from the substrate 14 to providethe sheet of aligned carbon nanotubes 22. During shaving, it isadvantageous to temporarily allow the sheet 22 to roll up over acylindrical, plastic roll. The sheet 22 may be subsequently unrolled forfurther processing or use. The etching process may be used, for example,where the substrate 14 is formed from metals. More specifically,hydrochloric acid may be used to dissolve the steel substrate and removeit without adversely affecting or damaging the carbon nanotube array 12,the polymer layer 20 or the flexible support 18.

The sheet 22 of aligned carbon nanotubes illustrated in FIG. 3 may beused as a final product. Typically, the carbon nanotubes incorporatedinto the array 12 of the sheet 22 have an average length of betweenabout 50 microns and about 500 microns. Typically, the polymer matrixlayer 20 has an average thickness of between about 10 microns and about500 microns. Further, the sheet 22 has a density of between about 0.2and about 1.0 g/cc and includes between about 98 and about 60 weightpercent aligned carbon nanotubes and between about 2 and about 40 weightpercent polymer.

Alternatively, a continuous tape 30 of aligned carbon nanotubes (noteroll 32 of tape 30 supported on a support tube 33 illustrated in FIG. 6)may be made from the sheet of aligned carbon nanotubes 22 illustrated inFIG. 3. More specifically, the flexible sheet 22 may be slit into strips34 of a desired width W. The strips 34 are then aligned end-to-end asillustrated in FIG. 4 to form the tape 30. More specifically, theabutting ends 36 of the strips 34 are aligned on and held in place by anadditional, thin, continuous flexible support layer 38. In one possibleembodiment the support layer 38 is made of the same material as theflexible support 18. The support layer 38 is continuous and bridges theabutting ends 36 of the adjacent strips. By laminating the support layer38 to the aligned strips 34 with an appropriate adhesive, heat fusion orby other means, a continuous tape 30 is provided.

As illustrated in FIG. 5, for certain applications it could be desirableto add a peel ply 40 to the opposite or otherwise exposed face of thecarbon nanotube array 12. The peel ply 40 may be made from substantiallyany appropriate material including but not limited topolytetraflouroethylene, polyester, nylon, coated paper, coated fabric,silicone, wax, polyolefin, metal or rubber and mixtures thereof. Anadhesive layer 42 may be provided between the carbon nanotube array 12and the peel ply 40 in order to hold the peel ply in position. Theadhesive layer 42 may be made from, for example, thermosets, latexs,rubbers, acrylics, pressure sensitive adhesives, silicones or othernatural or synthetic adhesives and mixtures thereof.

The following example is presented to further illustrate the invention,but it is not to be considered as limited thereto.

EXAMPLE

This example relates to the production of films using an epoxy matrixwithin CVD-grown multiwall carbon nanotubes grown on a quartz substrateto approximately 100-500 microns in length. Apart from the multiwallcarbon nanotube (MWNT) synthesis, the method is simple,straight-forward, and effective, which is an important part of itsattractiveness.

1. Multiwall carbon nanotubes were grown, primarily on one side, ofquartz substrates (4″×36″×⅛″) to a thickness of between 100 and 500microns. The process used is generally outlined in U.S. Pat. No.7,160,531. The parameters of the process were varied to produce MWNTs oflonger length.

2. The cooled MWNT-covered quartz substrates were then laid flat withthe MWNT arrays facing up. A thin layer of pre-mixed (epoxy+hardener)was applied to a PTFE coated flexible sheet (pre-cut to completely coverthe exposed MWNT array), and quickly placed over the exposed MWNTs withthe wet-epoxy side face-down onto the exposed MWNT array. A weight wasplaced completely over the dry side of the PTFE sheet (facing up) topress the epoxy-into the MWNT array evenly.

3. The epoxy was then allowed to infiltrate into the MWNT. (This processlikely occurs very quickly and is accelerated by the capillary action ofthe interstices of the MWNT array.)

4. Without removing the PTFE sheet, the epoxy was partially cured byallowing it to react, at room temperature, for 5 days. This is commonlyknown as B-staging of the epoxy, after which the epoxy was a stickysolid. (B-staging can be sped up significantly by the addition of heat,or by changing the epoxy chemistry. However, if the cured epoxy isbrittle, it must be cooled quickly to prevent complete curing.)

A desirable feature of the matrix was that it be flexible enough tofacilitate the bending stresses applied during the removal of theinfiltrated array from the substrate. This was subtly addressed by onlyusing a thin layer of epoxy during infiltration. If a thick layer ofepoxy were allowed to soak into the array, completely wetting it, theresulting array would adhere to the substrate. The thin layer of epoxydoesn't completely wet the array, but imparts enough mechanicalintegrity to sufficiently hold it together. The epoxy-starved array isideally suited for application as inter-laminar through-thicknessheat-transfer material in epoxy matrix composites because it also servesto soak up excess epoxy used during the processing of the laminated orfilament-wound composites.

5. The B-staged epoxy infiltrated MWNT array was then removed from theunderlying quartz substrate in a single large piece by “shaving” it fromthe substrate. Here a sharp razor was held at a shallow angle to andagainst the quartz surface, and pushed forward, thus cutting theinfiltrated array free from the quartz in one large piece.

It should be noted that during the MWNT synthesis, the MWNT array isinherently bound to the quartz substrate beneath. Attempts to remove theun-infiltrated array from the quartz substrate result in breaking up ofthe array into tightly scrolled, discrete chunks. Similarly, as grownMWNT arrays on metallic substrates are tightly bound.

The resulting free-standing B-staged epoxy infiltrated MWNT arrays, werein a sheet form and flexible enough to be conformed over curved surfacesor cut into desired shapes by a shaped-cutter technique, or let flat. Afinal curing of the epoxy can then be administered by simply heating toapproximately 150° C. for 1 hr. This hardens the epoxy matrix,locking-in the desired shape of the film. It should be noted that theflexibility and softness of the B-staged films is desirable to impartthe films with the conformability necessary to tightly fit the interfacein which they are placed. A high degree of surface contact with littlevoid space is desirable for thermal conductivity through an interfacebetween parts (such as between a CPU chip and its heat sink).

Yet another aspect of the present invention is a method of increasingunidirectional heat conduction from a work piece. This method may bebroadly described as comprising the step of wrapping the work piece witha continuous tape of aligned carbon nanotubes. The method is illustratedin detail in FIGS. 7A and 7B. As illustrated in FIG. 7A, the methodcomprises removing the peel ply layer 40 from the tape 30 as the tape iswound around the work piece P with the carbon nanotube array layer 12engaging the work piece P. The tape 30 may be wrapped in a“filament-winding” manner so as to provide a continuous wound surfacewith edge to edge engagement of the tape and no overlap. As noted above,the tape 30 incorporates a B-staged polymer 20 and is flexible enough tobe conformed to the curved surfaces of the work piece P. After windingthe tape 30 over the surface of the workpiece P, the support layer 38 isremoved by unwrapping (see FIG. 7B) leaving the supports 18 exposed.This is done while leaving the strips of aligned carbon nanotubematerial 22 in position completely wrapped around the work piece P. Thewound work piece P is then subjected to a final curing of the polymer20. For example, where epoxy is utilized for the polymer 20 the finalcuring could be administered by simply heating to approximately 150° C.for 1 hour. This locks the tape 30 into a work piece conforming shapeproviding a tight interface and high degree of surface contact betweenthe carbon nanotube array 12 and the work piece P. By providing tightcontact and minimizing void space high thermal conductivity is provided.As a consequence, heat is rapidly conducted from the work piece Pthrough the carbon nanotube array. It should be appreciated that thework piece P may take the form of substantially any number of structuresincluding but not limited to a microprocessor chip, a rocket motorcasing, laser equipment, filament-wound composites, laminate composites,electronics components, fiber reinforced composites, sheet moldedmaterials, composite engine cowlings and planar interfaces between heatsources and heat sinks.

The foregoing description of the preferred embodiments of the presentinvention have been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Obvious modifications orvariations are possible in light of the above teachings. The embodimentswere chosen and described to provide the best illustration of theprinciples of the invention and its practical application to therebyenable one of ordinary skill in the art to utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. All such modifications and variations arewithin the scope of the invention as determined by the appended claimswhen interpreted in accordance with the breadth to which they arefairly, legally and equitably entitled. The drawings and preferredembodiments do not and are not intended to limit the ordinary meaning ofthe claims in their fair and broad interpretation in any way.

1. A method of producing an aligned carbon nanotube tape, comprising:preparing strips of aligned carbon nanotube; and splicing said strips ofaligned carbon nanotube together end-to-end on a flexible support toform a tape.
 2. The method of claim 1 further including covering saidtape with a peel ply so that said spliced strips are covered on a firstface by said support and on a second, opposite face by said peel ply. 3.The method of claim 2, including winding said covered tape into a roll.4. The method of claim 1, wherein said preparing step includes steps of:growing an array of aligned carbon nanotubes on a substrate; and fixingsaid carbon nanotubes in alignment.
 5. The method of claim 4, whereinsaid fixing step includes steps of: coating a first face of a flexiblesupport layer with a polymer; placing said first face of said flexiblesupport layer coated in polymer over an exposed face of said alignedcarbon nanotubes in order to infiltrate said array of aligned carbonnanotubes on said substrate with a polymer; allowing said polymer topartially cure; and removing said flexible support layer.
 6. The methodof claim 5, wherein said preparing step further includes steps of:slitting said set polymer into strips of width W; and removing strips ofaligned carbon nanotubes of width W from said substrate.
 7. The methodof claim 6 wherein said removing step includes etching away saidsubstrate.
 8. The method of claim 6 wherein said removing step includesshaving said strips of aligned carbon nanotubes from said substrate. 9.A tape of aligned carbon nanotubes, comprising: a nanotube layerincluding aligned carbon nanotubes held in a polymer matrix; and a peelply covering a face of said nanotube layer.
 10. The tape of claim 9,wherein said aligned carbon nanotubes are multiwalled nanotubes.
 11. Thetape of claim 9, wherein said polymer matrix is made from a materialselected from a group consisting of a thermosetting resin, athermoplastic resin, epoxy, vinyl ester, silicone, cyanate ester,bismaleimide (BMI), polyimide, polyolefin, polyurethane, phenolics,acrylics, polyester; a carbonizable resin, polyfurfural, pitch, tar,ceramic matrix material, metallic matrix material, silicon carbide,aluminum, solder, rubber and mixtures thereof.
 12. The tape of claim 11,wherein said peel ply is made from a material selected from a groupconsisting of polytetrafluoroethylene, polyester, nylon, coated paper,coated fabric, silicone, wax, polyolefin, metal, rubber and mixturesthereof.
 13. The tape of claim 12, further including an adhesive layerbetween said nanotube layer and said peel ply.
 14. The tape of claim 13,wherein said adhesive is selected from a group consisting of athermoset, a latex a rubber, an acrylic, a pressure sensitive adhesive,a silicone and mixtures thereof.
 15. A method of increasingunidirectional heat conduction from a work piece, comprising: wrappingsaid work piece with a continuous tape of aligned carbon nanotubes. 16.The method of claim 15, wherein said continuous tape includes a supportlayer, a nanotube layer and a peel ply layer and said method includessteps of: removing said peel ply layer from said tape; winding said tapearound said work piece with said nanotube layer wound around said workpiece; and unwrapping said support layer from said nanotube layer woundaround said work piece.
 17. The method of claim 16 including providingan adhesive between said nanotube layer and said work piece.
 18. Themethod of claim 16 including curing said continuous tape following saidunwrapping of said support layer.
 19. The method of claim 15 includingselecting said work piece from a group of structures including a rocketmotor casing, a microprocessor chip, laser equipment, a filament-woundcomposite, a laminate composite, an electronics component, a fiberreinforced composite, a sheet molded material, a composite enginecowling and a planar interface between a heat source and a heat sink.20. A method of producing a flexible aligned carbon nanotube sheetmaterial, comprising: growing an array of aligned carbon nanotubes on asubstrate; infiltrating said array of aligned carbon nanotubes on saidsubstrate with a polymer; allowing said polymer to partially cure; andremoving said flexible aligned carbon nanotube sheet material from saidsubstrate.
 21. A flexible sheet of aligned carbon nanotubes, comprising:an array of aligned carbon nanotubes held in a partially cured polymermatrix material wherein said carbon nanotubes have an average length ofbetween about 50 microns and about 500 microns, said polymer matrix hasan average thickness of between about 10 microns and about 500 micronsand said flexible sheet has a density of between about 0.2 to about 1.0g/cc and includes between about 98 to about 60 weight percent alignedcarbon nanotubes and between about 2 and about 40 weight percentpolymer.
 22. The sheet of claim 21 wherein said polymer matrix is aB-staged epoxy.